Data storage devices including, e.g., those normally provided as part of, or in connection with, a computer or other electronic device, can be of various types. In one general category, data is stored on a rotating (or otherwise movable) data storage medium and a read head, a write head and/or a read/write head is positioned adjacent desired locations of the medium for writing data thereto or reading data therefrom. The head may include separate or integrated read and write elements. One common example of a data storage device of this type is a disk drive (particularly the type often called a “hard” disk or “fixed” disk drive).
Typically, information is stored on each disk in nominally concentric tracks, which are divided into sectors. The read/write head (or transducer) is mounted on an actuator arm capable of moving the head to access various radial positions of the disk. Accordingly, the movement of the actuator arm allows the head to access different tracks. The disk is rotated by a spindle motor at a high speed, which allows the head to access different sectors on the disk.
Although many concepts and aspects pertaining to the present invention will be described in the context of a disk drive, those with skill in the art, after understanding the present disclosure, will appreciate that the advantages provided by the present invention are not necessarily limited to disk drives.
In an idealized drive configured with nominally concentric data tracks, if a read/write head is kept a constant radial distance from the (nominal) axis of rotation, there will be no change in the distance (if any) from the read/write head to the desired data track, as the disk rotates. In actuality, however, many factors can contribute to deviations from this ideal condition such that small tracking correction forces must be applied to the read/write head to maintain the head sufficiently aligned with a desired data track as the disk rotates (although some amount of tracking error can be tolerated). Most modern disk drives provide a servo tracking system used for seeking to a target track and/or making tracking corrections to assist in maintaining tracking within acceptable ranges.
Typically, as part of a manufacturing or setup procedure (prior to normal use for data read/write), a hard disk drive is provided with a plurality of servo “bursts.” The purpose of these bursts is to provide location information to components of the head-positioning and/or tracking system. The present invention can be used in connection with any of a number of servo burst systems, and those with skill in the art will understand how to use the present invention using any of a number of servo burst systems at least after understanding the present disclosure. Generally, a plurality of servo bursts are positioned around the track. Typically, the bursts are circumferentially aligned, from one track to the next, over at least part of the radial extent of the disk, defining a plurality of servo wedges.
In a typical disk drive system, a disk drive responds to a data read/write request by determining the location of the target data (or, generally, of the initial portion or block of requested data). The location of the target data can be described by the target track, and the target sector, along that track. The disk drive then performs a seek operation intended to position the read/write head over the target track. A number of systems are used for performing seek operations, including those based on monitoring the relative position of the head (e.g., on the basis of the number of “tracks to go”) and obtaining appropriate control signals, e.g., from a table (which may be termed “position mode seek”) and providing control signals intended to achieve an acceleration, deceleration and/or velocity profile configured to reach the target track (which may be termed “velocity mode seeking”). At the end of the seek operation, the head will be relatively close to the desired center line of the target track and the servo-tracking system is then used for maintaining the read/write head on-track.
Typically, however, after the seek operation, the head will continue to move with respect to the target track center (owing to such factors as seek over-shoot, inherent stiffness in the servo system, and the like) until the tracking system oscillates or otherwise “settles” i.e. the head remains sufficiently (consistently) close to the track center that data read/write operations can reliably proceed. In general, it is desirable to provide a system with a relatively short settling time, e.g., to increase the likelihood that, following the seek operation, disk rotation will bring the target sector under the read/write head in less than one revolution. If the target data is brought, (by disk rotation) past the location of the read/write head during the settling process (i.e., before head radial position is sufficiently stable to permit read/write operations to commence), it will be necessary to wait until the head has settled and then wait an additional period of time required for another (full or partial) revolution of the disk to bring the target sector of the target track under the read/write head. On average, systems having a larger settling time can increase the number of “blown revs” (additional disk revolutions caused by the settling time). Relatively long settling times not only can contribute to undesirably low data throughput or other performance factors during normal operation, but can also contribute to disqualifying entire disk drives during a self-test operation, potentially contributing to a decrease in the effective productivity of a manufacturing line.
In a typical disk-drive system, for any given track location of the read/write head, there will be some amount of lateral force acting on the actuator arm and/or head tending to move the head off-track, which may be referred to as a “bias force.” Without wishing to be bound by any theory, there are a number of aspects of a disk drive which may contribute to bias forces, including, e.g., force imparted by a flexure connector, forces associated with actuator arm bearings, windage, the effect of a magnetic latch, and the like.
In a typical disk-drive system, during track-following, the head is maintained on-track by a servo-system which provides current to the voice coil motor having a magnitude and direction configured to maintain the head on-track, despite departure of the track from perfect centricity (including as a result of “runout” and/or bias forces). In at least some configurations, a servo-tracking system monitors the magnitude of the tracking error (e.g., by obtaining the “position error signal” or PES) and calculates the head-positioning control signals which will tend to move the head towards and/or maintain the head at an on-track position. Those with skill in the art will understand various ways of configuring such a servo-tracking system. Thus, for any given target track, the control signal will typically include a component which corresponds to compensating for the bias force, as well as other components such as components corresponding to compensating for runout, and the like.
In at least some configurations, the controller uses information indicative of the general magnitude of the bias component (and/or components) which may be obtained, e.g., from a table of stored values. The controller may combine various components (e.g., bias and runout) and adjust the result of combining these components, on the basis of the PES signal, to arrive at a final track-following signal.
With respect to the bias compensation table, various schemes have been used in attempting to provide appropriate bias compensation values including using tables which are indexed by target track location, recent seek directions, temperature, and the like.
Although previous approaches, including those described herein, have been useful in providing operable tracking systems, it is believed that there remains certain problems and areas for improvement with respect to bias compensation. For example, it is believed that the actual bias force operating on the head includes a transient component, i.e., even though, at two different points in time, or for two different I/O operations, a disk drive may be retrieving the same value from the bias compensation table (e.g., both I/O operations are at the same track, previous seek direction and/or temperature conditions), the actual bias force on the head will differ somewhat from the force that would correspond to the bias compensation value obtained from the table. Thus, actual bias can be considered the sum of a non-transient component (which can be compensated using a table look-up value) and a transient component which may change from time-to-time and which, without wishing to be bound by any theory, may be substantially non-repeating. It is believed that, in general, transient bias forces are induced by movement and can occur even for very short seeks such as one-track seeks (e.g., when the change in a flex lead-induced force would be expected to be small). Without wishing to be bound by any theory, it is believed that substantial bias forces, particularly for small seeks, may be associated with solid friction effects in the actuator arm bearing.
In at least some previous approaches, the non-transient component of bias was dealt with by using a value from a look-up table and the transient component was dealt with by the servo-tracking system. It is believed, however, that relying on the servo-tracking system (to provide compensation for components of bias not handled by the bias compensation table value) contributes to certain undesirable tracking system features such as undesirably long settling time of the tracking system following a seek. In addition, it is believed that large bias force errors can lead to larger-than-expected seek times. Accordingly, it would be useful to provide a system, method and apparatus which can at least partially compensate for transient bias components or other bias force not handled by the bias compensation table system.
Another potential problem with table-based bias compensation is the relative immutability of the table values. Basing bias compensation exclusively on values permanently stored in a table, makes the system unable to adjust to changes in the disk drive that may occur over time (e.g., arising from wear, aging of components, or other factors). Further, those bias force components which change on a relatively rapid time scale, including some or all of the described transient components, may be infeasible to accommodate using a table-based system. Accordingly, it would be advantageous to provide a system, method and apparatus in which at least a component of bias compensation is not based on table values and/or can accommodate changing circumstances, preferably including circumstances which change on a relatively short time scale such as the order of magnitude of the average time between successive seeks.
In at least some previous systems, the track-following servo includes an integrator which, substantially continuously, updates a control signal (typically based on the PES) to maintain the head substantially on-track. In many configurations, it is believed the integrator bandwidth is too low to maintain the head within fine tracking limits when the bias force changes at a high slew rate. In one sense, it is the delta (or change in bias force error from seek-to-seek) which is most problematic, rather than the actual or average magnitude of the error. A large, but relatively constant, bias force error can be learned by the integrator and used for all future seeks, but seek-to-seek changes in bias force error cannot readily be accommodated using table values or other values which are permanently stored (or which change on a relatively long time scale).
In at least some configurations, at the end of the seek operation, the track-following system will operate using an “initial value” in the integrator. In many previous systems, the initial value for the integrator was not well correlated with the control signal being used at the end of the seek operation and, particularly, the component representative of bias. In some configurations, the integrator initial value is left unchanged from the integrator value used in the previous seek. Such configurations are believed to lead to undesirable lengthening of the settling time and to increase the potential for blown revs. Without wishing to be bound by any theory, it is believed that such configurations create a “step” change in the bias force component of the value used by the integrator and that such step changes contribute to undesirable lengthening of settling times. Accordingly, it would be useful to provide a method, system and apparatus which can avoid or reduce step changes or similar occurrences leading to undesirable lengthening of the tracking-servo system settling time, particularly bias force compensation step changes.